Performance Evaluation of Filter Shape Index Modulation for sub-THz Downlink NOMA Systems in the Presence of Gaussian Phase Noise

In this work, we explore the application of an index modulation technique for Non-Orthogonal Multiple Access (NOMA) systems in the context of sub-THz communications. The modulation in use is called Filter Shape Index Modulation (FSIM), and it was specifically designed as a single-carrier modulation for sub-THz and THz communications. Our downlink system consists of a single-antenna Base Station (BS) and two single-antenna NOMA users. The deployed channels are Additive White Gaussian Noise (AWGN) ones. Additionally, we consider the existence of Gaussian Phase Noise (PN) at both receivers. We compare the Bit Error Rate (BER) performance of our system with that of a two-user NOMA system that uses Quadrature Phase Shift Keying (QPSK) modulation. We present the effect of different levels of PN on the receivers, and we investigate the scenario of imperfect Inter-Symbol Interference (ISI) cancellation.